In urban areas in North America a typical electrical distribution system provides three phases. When non-linear and other harmonic generating loads are connected to a three-phase system, harmonic currents are fed back into the power supply and can cause many problems, such as reducing the electromagnetic compatibility of the loads and overheating of transformers.
In a three-phase power distribution system, the phases can be phase shifted relative to one another, to suppress or completely cancel many of the more harmful harmonic currents. However, in rural locations, and other remote locations such as oil fields, often the electrical distribution system provides only a single phase. In these cases the power supply can still be connected to non-linear three-phase harmonic generating loads, for example variable speed drives and the like. Such loads, when fed by a single-phase power supply, become single-phase loads and generate substantially higher harmonic levels than when fed by a three-phase system.
Typical harmonics generated by a single phase harmonic generating load are the 3rd, 5th, 7th, 9th, 11th, etc. In addition to higher harmonic levels, when connected to a single-phase power supply such loads consume substantially higher current and have a substantially higher level of DC bus voltage ripple. This results in increased losses in the system and requires that the harmonic generating drive be de-rated, i.e. larger than the optimum rated drive, so as to consume less current and reduce DC bus voltage ripple.
For example, FIG. 1 illustrates a typical current consumption waveform of a single-phase adjustable speed drive, showing the current spectrum and a table of typical harmonic levels. Those skilled in the art will appreciate that the level of harmonic currents generated by such a variable speed drive fed by a single-phase power supply is approximately twice as high as the level generated by a three-phase variable speed drive connected to a three-phase power supply.
Prior art systems for mitigating harmonic currents fall into six basic types:
1. Power factor corrected (PFC) power supplies: In these systems the rectified current is continually adjusted to smooth the current consumption waveform. An example is illustrated in FIG. 3. PFC's are relatively expensive devices and their applications are limited. Also, PFC's cannot be retrofitted for use with existing power supplies, and are not practical for use with large ASD's.
2. Active filters: These devices inject into the conductors between the power distribution system and the load, harmonic currents having a polarity opposite to those generated by the load, thereby neutralizing harmonic currents flowing into the power distribution system. An example is illustrated in FIG. 4. Active filters have many disadvantages, including high cost, poor reliability and poor dynamic characteristics. Active filters also are not practical for use with large ASD's.
3. Resonant L-C filters: L-C filters are commonly used in power systems, tuned to different harmonic frequencies to mitigate specific harmonic currents. An example is illustrated in FIG. 5. These devices present many problems which are well known to those skilled in the art, including high cost, poor effectiveness in low voltage systems and the tendency to cause the system to operate with a leading power factor. Further, because L-C filters are non-directional they are easily overloaded by untreated harmonic currents generated by other harmonic sources connected to the power distribution system (for example in a neighbouring facility), resulting in overloading and frequent failures of the filter's capacitor bank.
4. AC chokes: In this harmonic mitigating technique reactors are connected in series between the line and the load. An example is illustrated in FIG. 6a (without a core) and 6b (with a core). This technique is simple, reliable and relatively low cost, however it results in a high voltage drop across the reactors. To reduce the voltage drop one must reduce the choke reactance level, which commensurately reduces the effectiveness of the choke and substantially limits harmonic current mitigation.
5. Phase shifting systems: Different kinds of phase shifters are available which allow the creation of quasi-multiphase systems, reducing certain harmonic levels. Harmonic currents of targeted orders are cancelled or substantially reduced depending upon the selected degree of the phase shift. However, such systems are typically limited in terms of the number of harmonic orders which can be mitigated, and the degree of harmonic mitigation depends upon the extent to which harmonics produced by the various harmonic sources and their phase shift angles are identical.
6. A harmonic mitigating system with a multiple winding reactor and capacitors, having a crosslink circuit providing an inductance to attract harmonic currents, as described in U.S. Pat. No. 6,127,743 issued Oct. 3, 2000 to Levin, which is incorporated herein by reference. Such a harmonic mitigating system presents a high impedance for harmonic currents flowing toward the power supply, but a low impedance for targeted harmonic currents flowing through the crosslink. Thus, the majority of harmonic currents are diverted through the crosslink and isolated from the power supply. This system addresses a large number of harmonic mitigating problems, but also provides certain drawbacks as a result of high internal impedance of the harmonic mitigating device.
The prior art for two-phase to three-phase converters falls into two basic types:
1. Mechanical converters such as rotating or dynamo-electric machines (motor-generators) in which a single-phase motor drives a three-phase generator. This conversion method is expensive and provides low reliability and high losses. Moreover, such devices introduce substantial impedance into the circuit supplying the non-linear load. The result is a high level of voltage distortion and reduced electromagnetic compatibility of the loads.
2. Various types of inductor-capacitor (L-C) networks. Examples of prior art configurations of two-element L-C networks for converting single-phase to three-phase are shown in FIGS. 7A to 7K. Examples of configurations of three-element prior art L-C networks for converting single-phase to three-phase are shown in FIGS. 7L to 7P. Examples of configurations of single-element prior art L-C networks for converting single-phase to three-phase are shown in FIGS. 7Q to 7T.
None of these prior art phase converters has any significant effect on harmonic currents generated by non-linear loads, and in fact in many cases the phase converter increases voltage distortion, thus making the situation worse. High levels of harmonic currents create many problems in power supply and distribution systems, including increased total harmonic distortion level of the voltage, reduced electromagnetic compatibility of the loads, reduced liability of the equipment resulting in higher maintenance costs and increased down time of the system, increased power losses, reduced power factor and other problems that are well known to those skilled in the art.
The prior art does not provide a system which combines harmonic mitigation and phase conversion. Thus, in order to avoid both of these problems, prior art systems supplying power to non-linear loads require a harmonic mitigating device and a separate two-phase to three-phase converter (in the case of a single-phase power supply) or phase shifting device (in the case of a three-phase power supply).
To mitigate harmonic currents generated by harmonic generating loads in a three-phase system, frequently a 12-pulse rectification system is used. In such a system the rectifier input comprises two three-phase rectifier bridges connected to the power supply by a phase shifting device that provides a 30° phase shift between the voltages supplied to the bridges. As well known to those skilled in the art, this cancels harmonics of the 5th, 7th, 17th and 19th orders. A reduction of other levels of harmonic currents using the same phase shifting principles requires an 18-pulse or 24-pulse rectification system, with associated additional cost and space requirements.
Furthermore, this solution is not available in a single-phase system. An alternative option currently being used for feeding single-phase non-linear loads, as noted above, is to use a harmonic mitigating device and de-rate the three-phase equipment. For example, for a typical adjustable speed drive (ASD) a de-rating of 50% is typically used. This de-rating substantially increases the cost of the system, and requires the use of additional harmonic mitigating equipment that further increases the cost and space requirements of the power system.
It would accordingly be advantageous to provide a system which combines harmonic mitigation and phase conversion.